In recent years, an ink-jet recording method has found wide applications in the field of various kinds of graphic arts such as photography, various printing, marking and specific printing such as a color filter because of being able to form images simply and cheap. Particularly, it has come to be possible also to obtain image quality compatible with silver salt photography by utilizing a recording apparatus which ejects and controls fine dots; ink in which such as a color reproduction range, durability and ejection suitability have been improved; and exclusive paper in which such as ink absorption, color forming property of a colorant and surface gloss have been improved greatly. Image quality improvement of an ink-jet recording method of today has been achieved only when a complete set of a recording apparatus, ink and exclusive paper is prepared.
However, an ink-jet system which requires exclusive paper is problematic in respect to limitation of a recording medium and cost up of a recording medium. Therefore, many attempts to record on a medium, on which ink is transferred, different from exclusive paper by means of an ink-jet recording. Concretely, there are such as a phase-conversion ink-jet method utilizing wax which is solid at room temperature, a solvent-type ink-jet method utilizing an ink which is mainly comprised of a rapid-drying organic solvent and a UV ink-jet method in which an ink is cross-linked by ultraviolet (UV) light after recording.
Among them, a UV ink-jet method has been noted recently in respect to relatively low odor compared to a solvent-type ink-jet method, rapid drying property and capability of recording on a recording medium without ink absorption property; UV-curable ink-jet ink is disclosed, for example, in Japanese Patent Publication No. 5-54667, JP-A (hereinafter, JP-A refers to Japanese Patent Publication Open to Public Inspection) No. 6-20204 and Japanese Translated PCT Patent Publication No. 2000-504778.
It is known that the aforesaid UV ink can be divided mainly into a radical polymerization type and a cationic polymerization type. In an ultraviolet radiation curable ink-jet recording system, image quality, i.e. impinged dot diameter, is controlled by factors such as light exposure timing after ink impingement, illuminance of exposed light, energy, ink droplet size, photosensitivity of ink, surface energy, viscosity, wettability of substrates, impingement pattern, and error diffusion pattern. Specifically, factors which mainly affect image quality are photosensitivity, viscosity, and surface tension of ink, wettability of substrates, and exposure conditions. Of these, when ink is cured by radical polymerization, photosensitivity is adversely affected by polymerization inhibition due to exposure to oxygen. As a result, the aforesaid photosensitivity largely depends on ink layer thickness and illuminance of exposing light. On the other hand, when ink is cured by cationic polymerization, photosensitivity is largely dependent on humidity, as well as temperature.
When a radically polymerizable ultraviolet radiation curable ink is used, in order to minimize the polymerization inhibition due to exposure to oxygen, known are inventions in regard to monomers, initiators, and initiation aids which are free from oxygen inhibition, and methods in which purging is performed employing inert gases such as nitrogen.
When a cationically polymerizable ultraviolet radiation curable ink is employed, in order to minimize the humidity dependent effects, known is a method in which impinged ink is heated as disclosed in Japanese Patent Application Open to Public Inspection No. 2002-137375.
Known as cationically polymerizable monomers employed in the cationically polymerizable ultraviolet radiation curable ink are epoxy compounds having oxirane ring(s), oxetane compounds, and vinyl ether compounds.
In particular, it is known that by using epoxy compounds together with oxetane compounds, the polymerization rate increases markedly. For example, pertinent publications include Toa Gosei Kenkyu Nenpo (Toa Gosei Annual Research Report) TREND No. 2 (1999), “Oxetane Kagobutsu no Hikari Cation Koka System eno Oyo (Application of Oxetane Compounds to Cationically Photocurable System of Oxetane Compounds” and Japanese Patent Publication No. 2679586”. Specifically, since oxetane compounds result in excellent heat resistance, adhesive properties, and chemical resistance, it is useful to simultaneously use these epoxy compounds which enhance reactivity.
As an application example of the aforesaid technique, Japanese Patent Application Open to Public Inspection No. 2001-220526 discloses an application to ultraviolet radiation curable ink-jet printing. In recent years, the ultraviolet radiation curable ink-jet recording system, in which ink is cured by ultraviolet radiation, has received attention as an image forming method employing substrates with less ink absorbability.
Generally, well known and used in practice as ultraviolet radiation curable ink is radically polymerizable ink. On the other hand, cationically polymerizable ink exhibits advantages such that polymerization inhibition due to oxygen, as found in the radically polymerizable ink, does not occur, low illuminance light sources are usable, unpleasant odors usually generated by acryl monomers are not formed, and components are low irritant, however, has not been put into practical use.
Listed as reasons for not being in practical use are properties in which photosensitivity markedly decreases under high humidity and is temperature dependent. Ambience dependent ink exhibits substantial problems in which its image quality varies depending on various ambience.
Japanese Patent Application Open to Public Inspection No. 2002-137375 discloses a method in which cationically polymerizable ink is employed, and impinged ink is heated and subsequently exposed to radiation. However, from the viewpoint of printer cost and use of low heat resistant components, the use of heating mechanism seems to not be preferable. Further, Japanese Patent Application Open to Public Inspection No. 2001-181386 discloses 2-(4-metoxyphenyl)-3,3-dimethyloxetane as a compound to increase the reaction rate while employing oxetane compounds. Said compounds, when employed together with oxetane compounds such as di[1-ethyl(3-oxetanyl)]methyl ether and alicyclic epoxy compounds, exhibits reactivity which makes it possible to replace the alicyclic epoxy compounds. However, when low illuminance light sources such as a fluorescent lamp are employed, the resulting photosensitivity under high humidity ambience has not been sufficient.